There are times when you see that someone needs some type of item or gold and you'd like to help. An obstacle could come in the form of time zones, or just being too lazy to try to log on when they're online or vice versa. Mailboxes would eliminate these issues.
This could be an addition to the guild hall/house. The player taps the mailbox and a window would pop up with different slots for the different messages. It doesn't have to just be items, they could be actual messages with words. Tap a tab to create a new message, tap another tab to add gold/items, or just tap on the chat window or chat box to type your message.
When a player gets a new message, it would come up as a push notification like with the nightly rewards, or an in game notification, that would give you the choice of transporting you to your mailbox. if a player is that concerned over not being scammed, then they could go through the trouble of setting up a time with the the're player and meeting up in game. The mailbox feature is to be an extension of the Giving system, not trading.